As part of the move to the Anaconda compilers we need to replace some parts of our tooling whilst not affecting the rest of the conda-forge build stack.

To do this we will publish to two labels (main and gcc7)

Implmentation of this is done in phases to avoid breaking the majority of the conda forge stack.

As part of this we are introducing a few system keys into conda-forge-pinning

• channel_sources

  This is a list of channels that are allowed to be used as sources for building conda-forge packages

  channel_sources:
  - conda-forge,defaults              # [compiler_label < 7]
  - conda-forge/label/gcc7,defaults   # [compiler_label >= 7]

• channel_targets

  This is a list of channels and labels that packages should be published to:

  channel_targets:
  - conda-forge main   # [compiler_label < 7]
  - conda-forge gcc7   # [compiler_label >= 7]

• build_number_decrement In order to ensure that post label merge we can have both sets of packages co-existing together in the same label, we will decrement the build number for packages built with the older compilers so that the new ones will have higher solver priority.

  build_number_decrement:
  - 1000              # [compiler_label < 7]
  - 0                 # [compiler_label >= 7]

• Adapt conda-smithy so that it can emit the required configurations for both compilers.

  PR: conda-smithy#836

• Adapt conda-force-ci-setup so that it can more easily be used to differentiate between compiler variants

  PR: conda-forge-ci-setup#19

• Adapt conda-forge-pinnings to refer to the new compilers and the new label

  PR: conda-forge-pinning-feedstock#92

• Create a migrator and subgraph using the conda-forge bot infrastructure on regro to carry out performing the rebuild migrations. The subgraph will include python 3.7, openblas 0.3.2, r-base 3.5.1, and all build inheritors from the compiler stubs.

  This migrator will use the conda-smity and conda-forge-pinnings merged during Phase 1

• Run the migrator against the stack. This will take several months to get to completion. conda-forge/core will monitor the progress of the migration and vote on when to move to phase 3

• Unify the two labels under the same one

• Label pre-gcc7 artifacts as pre-gcc7.

• Move gcc7 artifacts into main label.

• Remove support for building with toolchain_X

• There is an increasing large ABI incompatible gulf between defaults and conda-forge. In order to prevent user confusion and frustration it is best if we were more closely aligned with AnacondaRecipes

• Our existing compilers are very old and don't allow compilation against C++17 or other more modern C++ features

• This is a backwards incompatible change for ALL packages that are recompiled.
• We will not place the new packages in the main channel until the final phase of implementation.

All CFEPs are explicitly CC0 1.0 Universal.